1. Field of the Invention
This invention relates to code division multiple access communication systems and to methods for operating them, particularly to such systems and methods in a fixed wireless loop environment.
2. Discussion of the Related Art
Code Division Multiple Access (CDMA) has become one of the major technologies for digital wireless communication in the U.S. and worldwide. Growing demand for the service provided by fixed wireless loop systems based on CDMA exists particularly in those countries and areas that are insufficiently supplied with basic telephone infrastructure. In particular, the demand has grown for telephone connections of sufficient bandwidth and quality for convenient access to the Internet.
In systems addressing this need, the users are static and their locations are known. In this environment, digital wireless communication is provided in a fixed arrangement of cells containing fixed terminals. As an example for such a fixed wireless loop CDMA system, the code-limited capacity may be 128 channels, minus those used for pilot and control channels. For implementation, several options exist. First, the base station of each cell could use an omnidirectional antenna. Second, the cell can be divided into a number of sectors with a base station antenna for each sector. Codes can be reused in non-interfering sectors, but the inevitable overlap of the antenna patterns leads to a further requirement to avoid mutual interference between adjacent sectors. Thus, the third and fourth options pertain to multiple-sector cells and involve, respectively, using different portions of available bandwidth in adjacent sectors subject to mutual interference, or using different ones of the available codes in adjacent sectors subject to mutual interference. Further growth in demand currently can be met only by increasing the number of cells and the number of relatively expensive base stations. It is desirable to have a better way to meet increases in demand in such a CDMA system.
According to the invention, in a fixed wireless loop CDMA system having multiple sectors, there are established two protection sectors at both edges of each sector, The purpose of the protection sectors is to prevent mutual interference between adjacent sectors resulting from the roll-off of the radiation patterns of the sector antennas. The remaining part of a sector separates the protection sectors and will be called a basic sector. The spreading codes are divided among all sectors so that two neighboring sectors, no matter whether a protection sector next to a protection sector, or a protection sector next to a basic sector, do not have any codes in common. The two protection sectors on opposite edges of the same sector use codes from the same group of codes. In cells with an even number of sectors, three groups of codes are sufficient, one group for the basic sectors and two groups for the protection sectors. Cells with an odd number of sectors need four groups of codes, one for the basic sectors and three for the protection sectors.
Advantageously, all of the codes not used in the protection sectors are used in any one of the basic sectors.
Specifically, in a system according to a first aspect of the invention, each of the terminals in a protection sector or a basic sector is configured to use an assigned specific code from an appropriate one of the groups of codes.
Further specifically, in a method according to a second aspect of the invention, codes from appropriate ones of the groups of codes are assigned to terminals in protection sectors and basic sectors
Protection sectors are relatively narrow but broad enough to counteract the potential mutual interference between sectors.